Mobile electronic communication devices including cellular telephones, pagers, smartphones, remote monitoring and reporting devices, and the like have dramatically proliferated with advances in the state of the art of wireless communication networks. Many such devices are powered by one or more batteries, which provide a Direct Current (DC) voltage. One challenge to powering electronic communication devices from batteries is that the battery does not output a stable DC voltage over its useful life (or discharge cycle) but rather the DC voltage decreases until the battery is replaced or recharged. Also, many electronic communication devices include circuits that operate at different voltages. For example, the Radio Frequency (RF) circuits of a device may require power supplied at a different DC voltage than digital processing circuits.
A DC-DC converter is an electrical circuit typically employed to convert an unpredictable battery voltage to one or more continuous, regulated, predetermined DC voltage levels, and thus to provide stable operating power to electronic circuits. Numerous types of DC-DC converters are known in the art. The term “buck” converter is used to describe a DC-DC converter that outputs a lower voltage than the DC source (such as a battery); a “boost” converter, also called a step-up, is one that outputs a higher voltage than its DC input.
Supplying power to an RF power amplifier of an electronic communication device is particularly challenging. The efficiency of an RF power amplifier varies with the RF signal amplitude. Maximum efficiency is achieved at full power, and the efficiency drops rapidly as the RF signal amplitude decreases, due to transistor losses accounting for a higher percentage of the total power consumed. The loss of efficiency may be compensated by a technique known as “envelope tracking,” in which the output of a DC-DC converter, and hence the voltage supplied to the power amplifier, is not constant, but is modulated to follow the amplitude of the RF signal. In this manner, at any given moment, the power supplied to the RF power amplifier depends on the amplitude of the signal being amplified.
FIG. 1 depicts the relevant portions 10 of the RF circuit of a mobile electronic communication device, some of which may for example be implemented on an RF ASIC within the device. Depicted in FIG. 1 are a baseband and RF transmitter circuit 12, a Power Amplifier (PA) 14 providing an RF transmission signal to an antenna 16, a Power Management Unit (PMU) 18 supplying a dynamically varying supply voltage to the PA 14, and a transmitter measurement receiver and baseband circuit 20. Note that the division and grouping of circuit elements in FIG. 1 is functional and reflects important concepts in the envelope tracking operation discussed here. In a given implementation, the RF transmitter circuit 12 and transmitter measurement receiver circuit 20 may be grouped together, and the baseband functions may be implemented separately, such as in a digital signal processor (DSP) or baseband ASIC.
Within the baseband and RF transmitter circuit 12, an RF signal U(t) is generated by a digital modulator 22. Alternatively for example, during calibration procedures describe herein—the RF signal U(t) may be generated by a test signal generator 24. In either case, the RF signal is conditioned by signal conditioning block 26, and then flows along two parallel processing paths. An RF signal processing path 28 conditions and prepares the RF signal for amplification for transmission. In parallel, an envelope tracking processing path 30 extracts and processes the amplitude envelope of the RF signal, providing the instantaneous signal amplitude to the PMU 18 for the dynamic generation of supply power to the PA 14.
In the RF signal processing path 28, the gain of the RF signal is controlled in gain control block 32, and its delay is controlled in delay control block 34. The RF signal is further conditioned in signal conditioning block 36, and the In-phase and Quadrature components are separately converted to the analog domain by the IQ DAC 38. A low pass filter 40 isolates the RF signal from harmonics and other artifacts, and a mixer 42 generates a modulated carrier signal by mixing the RF signal with the output of a local oscillator (not shown). The modulated carrier signal is amplified by the PA 14 for transmission from the antenna 16.
To optimize the efficiency of the PA 14, its instantaneous power supply level is matched to the RF signal amplitude by processing the RF signal in the envelope tracking processing path 30. An envelope extraction block 44 extracts the RF signal envelope, which is scaled by the envelope tracking scaler block 46. The RF signal envelope is pre-distorted to compensate for known non-linarites using a Look-Up Table (LUT) 48, and the envelope delay is controlled by a delay control block 50. The RF signal envelope is converted to the analog domain by the ET DAC 52. A low pass filter 54 isolates the RF signal envelope from harmonics and other artifacts, and the processed and delay-controlled RF signal envelope is output to the PMU 18. The PMU 18 dynamically adjusts the supply voltage output to the PA 14 based on the RF signal envelope, to maximize the efficiency of the PA 14.
In the transmitter measurement receiver and baseband circuit 20, a receiver block 56 receives an RF signal output by the PA 14, and which is transmitted to the antenna 16. The receiver block 56 performs low-noise amplification, filtering, frequency down-conversion, signal processing, and analog to digital conversion. The transmitter measurement receiver baseband block 58 demodulates and decodes the received signal, and provides digital samples to a digital baseband block 60 for analysis, such as processing received test signals during PMU 18 calibration.
The transmission operations, the PMU 18 must be calibrated in such a way that for all relevant scenarios the PA 14 will minimize distortion and maximize power efficiency. This calibration typically comprises two steps: first, the non-linear relationship between the PA 14 supply voltage and RF signal envelop is determined; then the supply voltage provided by the PMU 18 is time synchronized with the modulated RF signal as it reaches the PA 14. Calibration of the non-linear relationship between the PA 14 supply voltage and RF signal envelope is conventionally performed using an iterative tuning algorithm at a selected number of envelope points. Time synchronizing the PMU 18 supply voltage output with the modulated RF signal at the amplifier is conventionally performed using an iterative tuning algorithm minimizing third order distortion products.
However, Iterative algorithms require iteratively controlling hardware to collect and analyze data, which takes a long time to perform. The use of envelope points (i.e., an RF signal with constant envelope) can create thermal problems during calibration. It is not possible to directly estimate time error during time synchronization. Finally, non-time-related distortion will mask time distortion.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.